1. Field of the Invention
The embodiments disclosed herein relate to lateral, extended drain, metal oxide semiconductor, field effect transistors (LEDMOSFETs) and, more specifically, to an LEDMOSFET having tapered airgap field plates to achieve a relatively high drain-to-body breakdown voltage and a method of forming the LEDMOSFET.
2. Description of the Related Art
Generally, integrated circuit structures are designed with the following goals in mind: (1) decreasing device size; (2) increasing device performance (e.g., by increasing switching speed) and (3) decreasing power consumption. Device size scaling can lead to a corresponding decrease in device channel lengths and, thereby a corresponding increase in switching speed. However, device size scaling has its limits because short channel lengths can also lead to a number of undesirable “short-channel effects”. These short-channel effects include, but are not limited to, a reduction in threshold voltage (Vt), an increase in drain leakage current, punch through (i.e., diffusion of dopants from the source and drain into the channel), and drain induced barrier lowering (DIBL).
To overcome or at least reduce such short-channel effects, halos can be incorporated into field effect transistor structures. Specifically, halos are highly doped regions, which have the same conductivity type as the field effect transistor body and which are positioned on each side of the channel (i.e., on the source-side and the drain-side of the channel) at the interfaces with the source and drain, respectively. These halos reduce the presence of short channel effects (e.g., increase threshold voltage (Vt), reduce punch through, etc.) and the effectiveness of the halos is dependent upon the location, concentration, and confinement of the halo dopant. Unfortunately, halos with a relatively high dopant concentration can also cause a corresponding decrease in switching speed.
Consequently, field effect transistor structures have been developed that balance the need to reduce the short channel effects exhibited by a scaled device with the need for a faster switching speed. For example, one such field effect transistor structure is a lateral, extended drain, metal oxide semiconductor, field effect transistor (LEDMOSFET) that is asymmetric with respect to the source/drain drift region configuration (e.g., the drain drift region can be longer than the source drift region, if any, and can have a lower dopant concentration). Those skilled in the art will recognize that the source/drain drift regions are also often referred to source/drain extension regions. Optionally, an LEDMOSFET can also be asymmetric with respect to the halo configuration (e.g., a source-side halo only). Such an LEDMOSFET provides decreased source resistance, increased threshold voltage, decreased off current (loft), increased leakage at the source-to-body junction, decreased leakage at the drain-to-body junction, decreased drain-to gate capacitance and decreased drain-to-body capacitance and, thereby limits short channel effects without decreasing switching speed. Typically such transistors have a drain-to-body breakdown voltage (Vb) of 10-15 volts, making them suitable for use in many applications. However, there are applications that require transistors with higher drain-to-body breakdown voltages. For example, for switch applications, a Vb of greater than 20 volts may be required and, for micro-electronic mechanical (MEMS) applications, a Vb of 30-50 volts may be required.